Rock Icicles appear to crawl out of an orifice in the cavern ceilings, then drop down in an attempt to harm Samus Aran on contact. Their attack pattern is similar to that of the Zebesian Skrees in the manner of dropping down from above. Whether they successfully hit Samus or not, they will always land on the ground and escape the vicinity by digging through it. Rock Icicles appear to live in some sort of family group or hive, due to the seemingly infinite number that come out from a single orifice.
In Metroid II: Return of Samus, these odd, fish-like creatures exclusively live in caverns that formerly contained purple liquid, similar to Skorps. They seem very cautious, as killing a single Rock Icicle halts the others from crawling out.
These creatures reappear in the remake Metroid: Samus Returns and behave much in the way they did before. However, they no longer cease to respawn when Samus kills a single one. The Melee Counter is capable of killing one before it drops down.
This is the only non-Metroid creature on SR388 to go unmentioned in Metroid II's English and Japanese instruction manual, including official English walkthroughs, nor does it have any official artwork. This could suggest that the Rock Icicle was either a very late addition to the game, or something that was planned to be cut. Issue 31 of Nintendo Power erroneously refers to these as "Gravitts" on page 56: "Keep an eye open for Gravitts as you wind down this passage. They hang onto the ceiling and drop as you pass under them. You'll be able to avoid contact with them if you're quick."
Before 2017, the only occurrence of an official name of this creature (岩つらら, lit. "rock icicle") existed in Japanese Metroid II: Return of Samus guides. Despite its name, it is not a stalactite and is actually a fish-like creature. The translated English name "Rock Icicle" is reused in the Prima Games Metroid: Samus Returns Official Guide, but it is yet unconfirmed if this is the official internal designation.
- Inhabitants & Eco System of SR388 (p. 17)
- "Rock Icicles are worm-like creatures with barbs on either side. They live in holes in the ceiling and regularly drop out onto the ground. They aren't hostile and only do you harm if you happen to be under a burrow when they drop out. These creatures are excellent for recuperating resources in a tight situation."
- Walkthrough (p. 31)
- "These enemies are the lowest life-form on the planet and pose next to no threat to the observant bounty hunter. Often they act less as adversaries and more as sources for health-refilling orbs and ammunition. They spawn ceaselessly from their holes; sit nearby, gun aimed and ready, and fire away as they come out. It's a slow method, but SR388 is a dangerous place, and being low on health is common. Refilling your supplies off these little critters works in a pinch."
- It is unknown if these lifeforms ever became X Parasite hosts, as Samus did not encounter any Rock Icicles in Metroid Fusion.
- The Rock Icicle's behavior of tunneling into the floor after falling from the ceiling makes them specifically akin to the Skrees seen in Samus' second mission to Zebes, unlike those featured in all other 2D titles in the series where they explode.
- Their seemingly infinite number is a trait shared by organisms that live in hives and Air Holes in the Metroid series. However, only Rock Icicles have the caution of ceasing their activity after the death of a single member of their colony, although this unique trait is removed in Samus Returns.
- The Rock Icicle is the first enemy Samus comes across in the Surface of Samus Returns.
- In Return of Samus, Rock Icicles spawn out-of-bounds in Phase 1, where they cannot be seen during normal gameplay but can be heard if Samus stands nearby. If the game is modded to allow Samus to bypass the intended level boundaries, the Rock Icicles can be seen with incorrect sprite graphics. While it is unknown why Rock Icicles are located here, the graphical error is most likely due to Rock Icicles not being one of the normally-encountered enemies in Phase 1 (thus the level's loaded graphic set does not include their sprites).